Aircraft derive flight data information from airstream probes of various types. For example, probes have been devised for detecting the direction of an airstream relative to the flight attitude of the aircraft and used to determine the aircraft's angle of attack and/or angle of side slip. In this regard, one particularly popular type of airstream direction probe that has been widely used on aircraft in recent years involves a probe which is rotatable about its longitudinal axis and which extends laterally into the airstream.
The conventional probe most preferably has a conical geometry, but a cylindrical probe geometry is also possible. When the probe is embodied in a conical geometry, elongated entry ports for the pneumatic signals are arranged in the probe's conical surface, preferably in planes containing the axis of the probe so that the ports also converge in a direction outwardly toward the probe's free end, and the space within the cone being divided by a partition. These elongated ports are also circumferentially spaced apart by a predetermined amount (substantially less than 180.degree.) so that each presents itself to the airstream on opposite sides of a stagnation line (which is the imaginary line along the surface of the probe which is presented directly to the airstream). The probe will therefore automatically seek a balance point at which the pneumatic pressures are equal in the two ports. See, for example, U.S. Pat. Nos. 2,971,375; 2,995,930; 2,445,746; 2,701,473; and 3,514,997 (the entire content of each such patent being expressly incorporated hereinto by reference).
Aircraft also typically have a pitot-static system which detects impact (ram) pressure of the airstream and the ambient atmospheric pressure. The impact pressure is typically taken from a pitot tube, mounted parallel to the longitudinal axis of the aircraft and generally in line with the relative airstream. The atmospheric (static) pressure is typically taken from static ports which can be flush-mounted to the aircraft fuselage (and thereby not affected significantly by the relative airstream), or static ports associated with the pitot tube. The sensed ram and static pressures can thus be used to derive useful flight information, such as airspeed, altitude and vertical speed.
One problem associated with conventional pitot-static systems is that the impact pressure sensed by the pitot tube or like ram air sensing probe varies with the aircraft's angle of attack and/or angle of side slip. Thus, the instruments employed in conventional pitot-static systems typically need to be calibrated and/or the pressure signals must be conditioned using on-board computing equipment in order to obtain highly accurate flight data information (i.e., so as to prevent errors which are a function of the angle of attack and/or side slip). It would therefore be desirable if an aircraft airstream direction probe could not only be employed to determine an aircraft's angle of attack and/or side slip, but also be employed to determine the dynamic and static pressures of the airstream so that self-compensating pressure information could be obtained. It is towards providing such a probe that the present invention is directed.
Broadly, the present invention is embodied in a rotatable airstream direction probe which is additionally provided with a dynamic pressure sensing port positioned substantially midway between a pair of pneumatic sensing ports which are symmetrically positioned with respect to the probe's stagnation line. A set of pneumatic output ports may thus be provided each of which communicates with a respective one of the pneumatic sensing ports in the probe.
Thus, when the pneumatic pressures within the paired sensing ports are balanced, the pneumatic pressure at the output port(s) in communication with the sensing ports will be essentially at a pressure P.sub.1 which is a monotonic function of static (atmospheric) pressure over a wide range of airspeeds (e.g., from 0.1 Mach to supersonic speeds). The dynamic pressure sensing port, on the other hand, will be presented directly to the airstream when the pressures within the pneumatic sensing ports are balanced. As a result, a dynamic pressure output port which communicates with the dynamic pressure sensing port will exhibit a maximum airstream pressure P.sub.0 which is a monotonic function of pitot (ram) pressure over a wide range of airspeeds. These pressures P.sub.0 and P.sub.1 can thus be converted mathematically into actual pitot (ram) and static (atmospheric) pressures undiluted by any error dependent upon the aircraft's angle of attack (when a single probe is used), and side slip (when multiple probes are used). The output ports can thus be connected operatively to on-board aircraft instrumentation and/or flight directors to derive useful flight data such as airspeed, altitude and vertical speed information.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.